The definition of the Inverse Square Law; “a law stating that the intensity of an effect such as illumination or gravitational force changes in inverse proportion to the square of the distance from the source.” It sounds far more complicated than it really is. Greg from Lens Pro To Go breaks it down in this easy to understand tutorial.
This tutorial is made for cinematography. However, all of these principles translate to photography as well. Once you understand this law of light you’ll find that you’re better able to control the look and feel of your work.